The present invention generally relates to a color image pickup apparatus for correcting the sharpness of the luminance signal in a color image pickup system for taking out as different color signals for each of the horizontal lines the color information from the objects to be photographed with the use of a color image sensor having a color filter arrangement face.
In recent years, color image pickup apparatuses with the use of a color image sensor are being popularized quickly by the best use of the characteristics of the smaller size and the lighter weight, and by the advancement in the higher performance. The various types of automatic correctors are further equipped with as higher tendencies. Image pickup apparatuses which are capable of coping with any objects to be photographed are expected. By the proper adjustment of the sharpness thereof even in the image quality, the resolution feelings of the finishing screen are improved. Therefore, the automatic adjustment of the sharpness becomes important in the color image pickup apparatuses.
An example of the color filters of a color image sensor for taking out the color information from the photographed objects as different color signals for each of the horizontal lines with the mosaic shaped color filters being arranged on the front face is shown in FIG. 16. The conventional construction example of the luminance signal processing portion of the color image pickup apparatus is shown in FIG. 21.
In FIG. 21, the construction and operation thereof will be described. The signal to be obtained from the color image sensor 1 is guided into a luminance signal separating circuit 4 and a vertical sharpness correcting signal generating circuit 3. The outputs of both the circuits are added by an adding circuit 6 and the luminance signal with the sharpness of the vertical direction being emphasized therein is obtained. The vertical sharpness correcting signal generating circuit 3 is composed of, for example, one horizontal scanning period delaying circuit (hereinafter 1H delaying circuit) 81, a subtracter 82 for getting the difference signal between the 1H delaying circuit output and the signal not to be delayed by 1H, a low-pass filter (hereinafter referred to as LPF) 83 and an amplifier 84.
The details of the signal to be obtained from the color image sensor 1 and the processing of the luminance signal processing circuit will be described with the use of FIG. 17 and FIG. 20 with the color filters shown in FIG. 16 being arranged as the color image sensor 1.
When the nH line of an odd number field or the n'H line of an even number field is scanned in FIG. 16, the signal to be changed for each of such picture elements as shown in FIG. 17 (a), (c) is outputted from the color image sensor 1. Also, when the (n+1)H line of the even number field or the (n+1)'H of the odd number field is scanned, the signal to be changed for each of such picture elements as shown in FIG. 17 (b), (d) is outputted from the color image sensor 1. From the nH line or the n'H line, the luminance signal of 2R +3G +2B is obtained as the low-pass signal, the color difference signal of 2B-G is obtained as the high frequency modulating signal. From the (n+1)H line or the (n+1)'H line, the luminance signal of the 2R +3G +2B is obtained as the low-pass signal, the color difference signal of 2R-G is obtained as the high frequency modulating signal. The characteristic graph of FIG. 18 shows the relationship between the output level (signal level) of these signals and the size (input light level) of the input light of the color image sensor 1. The straight lines (a) and (b) of FIG. 18 respectively show the signals of the picture elements Ye and Mg of the (n+1)H line or the (n+1)'H line and the signals of the picture elements of Cy and G. One dot chain line (e) shows the average value of both the picture elements, namely, the low-pass signal (luminance signal). Also, the dot lines (c) and (d) of FIG. 18 respectively show the signals of the picture elements of the nH line or n'H line Cy and Mg and the signals of the picture elements of Ye and G. Also, one dot chain line (f) shows the average value of both the picture elements, namely, the low-pass signal (luminance signal). Therefore, the processing of the luminance signal separating circuit 4 basically becomes the processing for removing the high-pass modulating signal through the low-pass filter in the output signal of the color image sensor. As shown in the wave form chart in FIG. 20, the processing of the vertical sharpness correcting signal generating circuit 3 becomes the processing for removing the high frequency modulating signal so as to amplify the proper magnification through the LPF 83 in the difference signal (FIG. 20, the wave form (c)) between the output signal (FIG. 20, the wave form (a)) of the color image sensor 1 and the output signal (FIG. 20, the waver form (b)) of the color image sensor 1 delayed by one horizontal scanning period. Accordingly, the luminance signal with such vertical sharpness shown in FIG. 20, the wave form (d) being corrected therein is obtained into the output of the adding circuit 6 of FIG. 21.
But the conventional color image pickup apparatus of such construction as described hereinabove has the following problem points. The luminance signal to be obtained from the color image sensor 1 is an average signal of two picture elements adjacent to the respective horizontal lines, the value thereof becoming equal in each horizontal line in principle (unless the object to be photographed changes). For example, as the incident light quantity becomes large, the signal from the picture element for generating the large signal level out of the respective picture elements of the color image sensor begins its saturation, with a problem that the luminance signal level of the adjacent two horizontal lines does not conform to each other before all the signals of the other picture elements are saturated. This situation will be described in detail with the use of FIG. 18, FIG. 19, FIG. 22. In the characteristics graph of the above described FIG. 18, the luminance signal level (shown in one dot chain lines (e), (f) in FIG. 18) of the adjacent two horizontal lines agrees with each other from the saturation level S sat of the color image sensor 1 to the incident light quantity Ll where the signal level of each picture element is small. When the incident light quantity exceeds the Ll, the signals of the picture elements of the (n+1)H line or the (n+1),H line Ye and Mg shown in the straight line (a) of FIG. 18 are saturated at the S sat. On the other hand, the signals of the picture elements of the Cy and the G in the (n+1)H line or the (n+1)'H line of the straight line (b) of FIG. 18, and the signals of the picture elements of the Cy and the Mg of the nH line or n'H line shown in the dotted lines (c) and (d) of FIG. 18 and the signals of the picture elements of the Ye and the G are not saturated yet so as to increase the signal level as the light quantity is added. The nH line or n'H line luminance signal does not change in the addition of the signal level accompanied by the incident light quantity increase like a straight line (f) in FIG. 18, while the (n+1)H line or the (n+1)'H line luminance signal becomes smaller in the signal level increase accompanied by the incident light quantity increase like a straight line (e) in FIG. 18, thus causing the difference in the luminance signal level of two horizontal lines. FIG. 19 shows a signal wave form chart for each picture element of the respective horizontal lines. FIG. 19 (a) shows the signal level for each of the (n+1)H line or the (n+1)'H line picture element, with the larger one showing the signal of the picture elements of the Ye and the Mg. FIG. 19 (b) shows the signal level of each picture element of the nH line or the n'H line.
When the signals different at the average signal level in the adjacent two horizontal lines as described hereinabove are processed by the signal processing circuit of the conventional color image pickup apparatus of FIG. 21, further the signal level difference of two lines is emphasized as shown in FIG. 22. The wave form (a) of FIG. 22 is the output signal from the color image sensor 1, which has the signal level difference of the dS between the adjacent lines. The wave form (b) of FIG. 22 is a signal thereof delayed by 1H. The wave form (c) of FIG. 22 is a difference signal between the wave form (a) and the wave form (b), and becomes a vertical sharpness correcting signal. The signal level difference dS, between two lines of the output luminance signal corrected in the vertical sharpness shown in the wave form (d) of FIG. 22 becomes larger than the level difference dS in the input signal.
Although it is explained that the signal is saturated by the color image sensor 1, these circuits may be saturated earlier than the color image sensor 1 in, for example, the construction where the amplification circuit and the A/D converting circuit have been arranged immediately after the color image sensor 1 without restriction to this example. Even in these cases, the same problem as described hereinabove occurs.